The present invention relates to an improved adjustable bottom closure for a resonator on sound bar type musical instruments, and more particularly relates to improvements in construction of a bottom closure used for defining the effective length of an associated tubular resonator on sound bar type percussive musical instruments such as marimbas, xylophones and vibraphones.
Generally, a sound bar type percussive musical instrument is provided with a graduated series of sound bars each tuned to a prescribed resonance frequency, and different sound bars are associated with tubular resonator of different lengths each closed at the bottom. When struck by a mallet, each sound bar generates a sound wave which drives air in the associated resonator for resonance thereby increasing the tone volume.
The resonance frequency f in Hz of a resonator is given by the following equation; EQU f=(V/4l)-(correction.times.R/2)
where
V is the velocity of sound in m/sec, PA1 l is the effective length of the resonator in m, PA1 R is the inner diameter of the resonator, and PA1 the correction is a value in a range from 0.6 to 1.0. PA1 where t is the ambient temperature in .degree.C.
Here, the velocity of sound V is given by the following equation; EQU V=340 m/sec+(t-15).times.0.6 m/sec
From these equations, it will be well understood that the value of the resonance frequency f of a resonator is greatly swayed by the value of the ambient temperature. In contrast to this, the frequency of the sound wave generated by an associated sound bar at striking is influenced by the ambient temperature in a reversed fashion. In order to make up for this gap in temperature influence, the resonator needs to be put in tune in accordance with the ambient temperature for best resonance.
To this end, it is usually employed to provide a resonator with an adjustable bottom closure which enables adjustment in effective length of the resonator depending on its set position within the resonator. A typical conventional bottom closure is given in the form of a simple cup-shaped piece associated with a gasket bonded to its outer surface. In use, the cup-shaped piece is inserted into the bottom of a resonater with the gasket in contact with the inner wall of the resonator. When the bottom end of the resonator is more or less deformed, this conventional simple construction cannot well follow the deformed shape of the resonator bottom end, thereby developing gaps between the gasket and the resonator inner wall which allows undesirable air leakage at resonance. In addition, when the axial direction of the cup-shaped piece is even a little off that of the resonator, like gaps are tend to be developed between the gasket and the resonator inner wall which promote undesirable air leakage at resonance.
In order to avoid this trouble, a new bottom closure was proposed in Japanese Utility model Publication sho.56-38553. In the case of this proposed construction, a base plate is secured in a resonator near its bottom end and an adjuster rod is rotatably passed through a hole in the base plate while being blocked against axial diplacement. On the inner side, the point of the adjuster rod is in screw engagement with a mobile closure and, on the outer side, the adjuster rod is provided with a knob for manual operation. By manually turning the adjuster rod about its axis via the knob, the mobile closure slides up and down in the resonater for adjustment of the effective length of the resonator. In this case, the screw engagement of the mobile closure with the adjuster rod allows fine adjustment of the effective length and the axis of the closure can be always registered right at the axis of the resonator. Despite these advantages, this proposed construction cannot still provide good follow-up to possible deformation at the resonator bottom end. In addition, the proposed bottom closure is rather complicated in construction, which causes increased labour at installation and increased weight of the musical instrument. Further, the extent of displacement of the mobile closure, i.e. the extent of adjustment in effective length, to be provided by the screw engagement is too small to attain satisfactory tuning.